Homodyne laser radar (ladar) velocimeters, such as ground speed indicators for aircraft, detect the relative speed between a laser and a reflective target by exposing a photodiode simultaneously to a portion of the laser's light and to the return signal produced by the reflection of the laser's light from the target. Relative motion of the target with respect to the laser (or vice versa) produces a Doppler shift in the frequency of the reflected beam. the photodiode sees this shift as a light intensity variation at a beat frequency proportional to the speed of the relative motion. The resulting signal at the output of the photodiode circuit is the speed signal.
In order to prevent saturation of the photodiode by one of the two component signals it detects, the amplitude relationship between the local oscillator (LO) signal produced by the laser and the return signal reflected by the target must be carefully controlled. Also, the small diameter of the laser beam requires a very precise alignment of the optical components of the system; and, as in any airborne equipment, size and weight are important practical considerations.
In the prior art, it has been conventional to derive the LO signal from the transmitted laser beam through the use of a beam splitter which reflected a fixed percentage of the impinging laser beam and transmitted the rest. Consequently, in order to change the intensity ratio between the LO signal and the return signal at the detector, it was necessary to change the beam splitter and realign the system--something which could only be done at a maintenance facility.
Prior art systems relied, as does the inventive system, on the use of a polarization-sensitive optical element such as a Brewster plate to separate the transmit laser beam from the return signal beam. For this reason, the laser beam is usually linearly polarized. In the prior art, the polarization of the LO signal had to be shifted by 90 degrees after the LO signal was split off from the transmitted beam in order to match the polarization of the target signal at the detector. This required a half-wave retardation plate to be placed in the path of the LO signal at a location which caused the optical layout of the system to have a substantial width and length.
The prior art includes the following U.S. Pats. Nos.: Buhrer 3,215,840 and 3,435,229 using separate detectors for the LO and target signals; Dento 3,532,890 and Furukawa 3,584,221 relating to an optical pulse code modulated multiplexer; Chen 3,638,024 relating to pulse interval modulation of a laser beam by controlling polarization; Henning 3,694,656 dealing with a demodulation system; Graves et al 3,975,628 relating to a laser receiver immune to Doppler shifts and noise; O'Meara 4,011,445 dealing with a dual-receiver laser radar imaging system; Goodwin et al relating to a PCM communications receiver using quadrature demodulation of laser signals; Schlossberg 4,131,792 relating to a resonant diplexer; Stacy et al 4,439,014 relating to an electro-optic modulator; and Monerie et al 4,056,388 relating to a two-detector orthogonal demodulator. Overall, the cited prior art is mostly concerned with methods of mixing signals polarized in opposite directions.